A broad objective for this proposal is to reduce stress resulting from polymerization shrinkage in resin composites through a comprehensive characterization of its gelation behavior and evaluation of constituent contributions to its mechanical properties and residual stress state. This will be achieved through four specific aims. The first is to correlate post-gel polymerization contraction strain to total contraction strain. Parallel experiments with total and post-gel contraction measurement techniques will test the hypothesis that the ratio of post-gel to total (pre- and post-gel) contraction is lower in materials that exhibit lower contraction force, slower development of degree o conversion, and slower development of stiffness. The second is to evaluate gelation behavior and effective stiffness for composites during polymerization Dynamic mechanical analysis (DMA) will characterize the gelation behavior of composites for correlation with post-gel and total contraction data. This will provide the baseline from which to modify onset of gelation. The third is to evaluate methods for delaying the onset of gelation in the polymerization development continuum. The method of light application and the concentration o photosensitizer and reducing agent will be modified to test the hypothesis tha gelation can be delayed by altering the photoinitiation process thereby reducing shrinkage and residual stress, without sacrificing physical propertie of the composite. The fourth aim is to develop accurate, correlated numerical models of polymerization contraction using a micromechanics approach. Opportunities for reducing residual stress will be evaluated through a parametric analysis of constituent behavior using newly developed computer modeling algorithms. New tools will be developed to assess residual stress in current dental composite systems with current placement and curing methods so that future advances in polymer chemistry and curing techniques can be evaluated for their benefits. Clinically significant reductions in contraction strain and stress will be produced which will improve the function and longevity of resin composite restorations.